1. Field of the Invention
This invention relates to a method for controlling the bead size in the nip of glazing rolls used in the extrusion of thin-walled plastic webs having smoothed surfaces on both sides by continuous measuring of the birefringence in orientation at various points over the width of the extruded plastic web. This allows exact controlling of the throughput of the nozzle connected to the extruder, provided that the nip is fully constant over the width of the glazing rolls.
2. Background art
One of the main aims in producing extruded plastic webs, for instance from thermoplastic materials, is to achieve a thickness as uniform as possible and a homogeneous characteristic curve over the whole surface of the web. This applies in particular to plastic films i.e. plastic webs of a thickness of less than 2 mm.
The main reason for irregularities in plastic webs are variations over the width of the outlet gap of the melt stream emerging from the sheet die. Assuming a parallel nip between the glazing rolls, such variations in the melt stream inevitably result from differences in the height of the melt stock--in the following referred to as "bead"--in the roll nip. Differences in the bead size involve varying pressures in the roll nip and thus differing melt stream rates. This inevitably leads to differences in the thickness of the final product. In case of too big beads so-called bead rolls develop on the web surface and in case of no bead there is normally no contact between the plastic melt and the surface of at least one glazing roll. At such spots, the nominal thickness of the web falls short of the desired value. Constant controlling and adjusting of the bead during extrusion is of great assistance therefore. DE-OS 35 43 632 discloses a method using a time-limit relay to determine thickness and orientation of plastic films by means of polarized light.
Numerous publications deal with the determination of film thickness by the infrared absorption spectroscopy the principles of which are described by W. E. Van Horne (Tappi 58(4), pages 111 to 114, 1975). DE-OS 32 30 442 desribes a method which eliminates errors of measurements in the determination of film thickness by means of the infrared absorption spectroscopy which may be caused by inhomogeneities in the film material for instance. In Applied Optics 11(9), pages 1907 to 1915 (1972), P. A. Fluornoy et al. introduce a device to determine the thickness of films and their coatings by means of interferometry. Such device can be used to determine the thickness of freely running films by transmission or reflexion. The basic design includes a light force providing a continuous spectrum, the light of which is reflected on the front and rear sides of the film to be measured. The resulting phase difference which is proportional to the film thickness is determined in a Michelson interferometer.
A continuous and non-contact measuring method to determine the thickness of electrical non-conductive materials, in particular webs of polymer materials, is described in DE-OS 40 11 646. This method uses an air stream caused by an atmospheric pressure to detect a specific distance between the measuring probe and the material web.
DE-OS 36 31 652 discloses a measuring arrangement for a non-destructive and non-contact thickness determination of films and thin surface coatings by means of instationary heat conduction.
Another measuring device to determine the thickness of films and in particular of plastic films is described in DE-OS 38 43 300 and comprises, among other parts, a roll on which the film abuts over part of its circumference and a light ray which is glidingly and tangentially passed by that section of the roll where the film abuts, characterized in that the light ray behind the roll is measured over its total width by means of a light sensor. All aforementioned methods only determine the thickness distribution of a plastic web without allowing conclusions to the reasons.
Greek Document 85,1420 discloses a method and apparatus for the production of extruded and smoothed plastic webs by extruding a thermoplastic moulding material using a sheet die and smoothing the extruded melt web by forming a bead of the thermoplastic moulding material in front of the nip of the glazing rolls. The size of the bead is determined by measuring the web's surface temperature on leaving the roll nip. In case the nominal temperature of the film surface ensuring a constant operating condition in continuous operation is exceeded, the melt throughput will be increased and in case it falls short, reduced. This eliminates the surface interferences occurring with no bead or with varying bead sizes.
In G 92 08 837.6 apparatus is described to eliminate the influence of a roll which is out of truth on the control of the thickness distribution of an extruded surface-smoothed sheet. The characteristical features, traversing bead size measuring and/or traversing sheet thickness measuring, ensure a uniform bead profile at the glazing roll mechanism. This information is made use of to control the melt stream emerging from the nozzle.
The methods referred to as prior art to determine the thickness of extruded plastic webs or sheets do not allow direct use for an automatic control of the local melt stream emerging from the nozzle because the thickness variations measured therewith may have been caused by other reasons than melt stream fluctuations. They may, for instance, be a consequence of roll deflections caused by high pressures at the roll nip or by ovalities of a roll originating from production. The use of a sheet thickness measuring device to control the melt stream emerging from the nozzle is therefore most questionable when producing smoothed webs.
GR 85.1420 describes a method for controlling the nozzle by means of measuring the bead height. The measuring method used therein which determines the web surface temperature does not sufficiently detect very small differences in bead size as particularly common in the production of very thin-walled films, however. Because of the traversing way of measuring, possible thickness variations in the direction of extrusion caused, for instance, by an untrue glazing roll may moreover be misinterpreted as thickness variations over the width.
The method described in G 92 08 837.6 at least allows an elimination of the problem of thickness variations caused by glazing rolls which became deformed. The fact that the measurement is taken twice, offset by half of the roll circumference, and averaged allows a compensation of the sinus-shaped measurement fluctuations in the extrusion direction caused by deformed rolls.